Improve issue filtering for github

[texel-sensei]

Right now the github adapter filters issues via assignee. It uses the username of the repository url, but this only works for the repository owner.

Either:

1. Don't filter at all. Not sure how this would work out with repositories that have a huge number of issues.
2. Use the name/email from the git repository. However this might not be the same as for the github user.